1. Field of the Invention
The present invention relates to film resistors which are trimmed to set the resistance value of the resistor to a predetermined value and, more particularly, to thick or thin film resistors which are laser trimmed to remove a portion of the resistor body within and displaced from the edges of the resistor body.
2. Description of the Prior Art
Many methods for trimming film resistors are well known to those skilled in the art. The use of a laser has significantly improved the accuracy of the laser trimming procedure and has significantly reduced the time necessary to perform the calibration process.
German Patent 23-37-466, which issued in 1975, illustrates several different configurations of film resistors with material removed along a cut portion. All of the cut portions shown in this patent intersect an edge of the film resistor body, indicating that if a laser was used to perform the material removal operation the laser was initially energized at or outside an edge of the film resistor body. This technique is generally similar to those which will be described below.
U.S. Pat. No. 4,097,988, which issued to Hauschild on Jul. 4, 1978, discloses a method of manufacturing thick film resistors to precise electrical values. The resistance material is applied to a substrate in a layer which has a configuration that tapers in a direction that is transverse to the direction of flow of current. Conductive leads are provided in contact with the tapered edges of the layer. An incision on the shorter side of the layer, between the tapering edges to which the leads are applied, will have a relatively large effect on the resistance value without having a significant affect on the amount of the area that determines the power rating of the resistor. Therefore, trimming by such an incision can be done quickly and the resistance change with progressive incision is more uniform than in the case of a resistor layer of rectangular configuration.
U.S. Pat. No. 4,159,461, which issued to Kost et al on Jun. 26, 1979, describes a resistor network having extended trim ratio and improved trimming and operating characteristics. The resistor network comprises an insulated substrate which has at least one film resistor formed on it and a pair of opposed film conductor electrodes disposed on opposite side of the film resistor. Side edges of the film resistor are engaged by film conductor electrodes and flare outwardly from the bottom edge of the film resistor to terminate a dome shaped top region that is elongated and semicylindrical in configuration. The dome shaped top region is not engaged by the film conductor electrodes and the film resistor is trimmed by removal of a notch from the film resistor starting from the bottom edge and extending upwardly along the line substantially centered beneath the apex of the dome.
U.S. Pat. No. 4,041,440, which issued to Davis et al on Aug. 9, 1977, describes a method of adjusting resistance of thick film thermistor. The thermistor-resistor network disclosed in this patent is functionally adjusted and has an accurately preselected electrical resistance and preselected change of electrical resistance as a function of temperature. The network features a thermistor and/or a conductor segment contacting the thermistor having plurality of parallel paths. Some of the paths preferably provide a different resistance change with temperature than the other paths. A selected number of the parallel paths are severed during functional testing to adjust the thermistor to a preselected change in network resistance with change in temperature. The resistor is functionally adjusted in the usual manner to obtain the selected total network resistance value at room temperature.
One particular method of performing a laser trimming operation on a film resistor is to direct the laser beam at a position outside the resistor body, but at a known distance from a preselected edge of the resistor body. The laser is typically energized when the beam is pointing at this first position and, after energization, the laser beam is moved toward the resistor body to remove material from the laser body along the path traveled by the laser beam. The material removed from the film resistor body is typically equal to a width determined by the diameter of the laser beam. As the laser continues to move inward from the edge of the resistor body, more material is removed. At any point during this procedure, the laser can be deenergized and the effective resistance of the film resistor can be measured empirically to determine the necessity for further trimming. If the resistance is lower than desired, the laser can be reenergized and the movement of the laser beam can be continued from the position at which it was stopped during the measurement operation. After further movement and further removal of resistor material, the laser can again be deenergized to permit the resistance to again be measured. This procedure can be repeated until the proper resistance value is achieved. This start and stop process is sometimes referred to as "nibbling" with the overall procedure sometimes requiring numerous deenergizations of the laser and measurements of the resistor to achieve the appropriate resistance value.
A slightly modified technique also begins with the laser beam directed to a point outside the body of the resistor. However, the required travel of the laser beam is precalculated and theoretically determined prior to energization of the laser and movement of its beam. The laser beam is energized when the beam is directed at a position which is outside the body of the film resistor. The distance from the beam to the nearest edge of the resistor is known, within the limitations imposed by the equipment and operational tolerances. The required length of the cut to the resistor body is theoretically calculated and the distance of travel of the laser beam is therefore generally equivalent to the sum of the length of the material removal cut and the distance between the original position of the laser and the edge of the resistor body. This theoretical approach provides a significant improvement over the "nibbling" technique described above, but several serious problems remain.
When the laser is used to trim a film resistor body in the manner described immediately above, it is necessary that the original position of the laser and its distance from the resistor body edge be known with a high degree of accuracy. If this dimension varies from its assumed value, the length of the cut made by the laser into the resistor body will vary and, therefore, the resistance of the film resistor will vary. For example, if the original position of the laser beam is farther from the nearest edge than anticipated the overall travel of the laser beam will result in a cut in the resistor body which is shorter than expected. On the other hand, if the original position of the laser beam is closer to the nearest edge of the resistor body than expected, the predetermined travel of the laser will result in a cut in the resistor body which is longer than expected. Because of this uncertainty, the first pass of the laser must be slightly shorter than the mathematically calculated length. This prevents the possibility that the cut be made too long on the first pass since this type of error is not correctable. Therefore, after an initial pass by the laser beam, to a position shorter than actually desired, the laser can be deenergized and the resistance of the resistor can be empirically determined by measurement so that the second energized travel of the laser can be mathematically calculated with a high degree of accuracy. However, because of the uncertainty in the initial position of the laser, a minimum of two steps is required to perform this technique.
It would therefore be advantageous if a laser trimming procedure could be developed which permits the resistor to be accurately trimmed with a high degree of confidence but requiring only a single energization and deenergization of the laser and a single episode of travel to perform this function.